1. Field of the Invention
Generally, the present disclosure relates to microstructures, such as advanced integrated circuits, and, more particularly, to material systems dielectrics having a low dielectric constant, such as silicon oxide-based dielectrics.
2. Description of the Related Art
When fabricating modern microstructures, such as integrated circuits, there is a continuous drive to improve performance in view of operational behavior and diversity of functions integrated in a single microstructure device. For this purpose, there is an ongoing demand to steadily reduce the feature sizes of microstructure elements, thereby enhancing performance of these structures. For instance, in modern integrated circuits, minimum feature sizes, such as the channel length of field effect transistors, have reached the deep sub-micron range, thereby increasing performance of these circuits in terms of speed and/or power consumption and/or diversity of functions. As the size of individual circuit elements is reduced with every new circuit generation, thereby improving, for example, the switching speed of the transistor elements, frequently new materials may be required in order to not unduly offset any advantages that may be achieved by reducing the feature sizes of the individual components of microstructure devices, such as circuit elements and the like. For instance, upon shrinking the critical dimensions of transistors, thereby increasing the density of individual circuit elements, the available floor space for interconnect lines electrically connecting the individual circuit elements is also decreased. Consequently, the dimensions of these interconnect lines are also implemented with reduced dimensions in order to compensate for a reduced amount of available floor space and for an increased number of circuit elements provided per unit die area as typically two or more interconnections are required for each individual circuit element. Thus, a plurality of stacked wiring layers, also referred to as metallization layers, is usually provided, wherein individual metal lines of one metallization layer are connected to individual metal lines of an overlying or underlying metallization layer by so-called vias. Despite the provision of a plurality of metallization layers, reduced dimensions of the interconnect lines are necessary to comply with the enormous complexity of, for instance, modern CPUs, memory chips, ASICs (application specific ICs) and the like.
Advanced integrated circuits, including transistor elements having a critical dimension of 0.05 μm and even less, may, therefore, typically be operated at significantly increased current densities of up to several kA per cm2 in the individual interconnect structures despite the provision of a relatively large number of metallization layers owing to the increased number of circuit elements per unit area. Consequently, well-established materials, such as aluminum, are being replaced by copper and copper alloys, i.e., materials with a significantly lower electrical resistivity and improved resistance to electromigration even at considerably higher current densities compared to aluminum.
The introduction of copper into the fabrication of microstructures and integrated circuits comes along with a plurality of severe problems residing in copper's characteristics to readily diffuse in silicon dioxide and other dielectric materials, as well as the fact that copper may not be readily patterned on the basis of well-established plasma-assisted etch recipes. For example, based on conventional plasma-assisted etch processes, copper may not substantially form any volatile etch byproducts, such that the patterning of a continuous copper layer with a thickness that is appropriate for forming metal lines may not be compatible with presently available etch strategies. Consequently, the so-called damascene or inlaid process technique may typically be applied in which a dielectric material is formed first and is subsequently patterned in order to receive trenches and via openings, which are subsequently filled with the copper-based material by using, for instance, electrochemical deposition techniques. Moreover, copper has a pronounced diffusivity in a plurality of dielectric materials, such as silicon dioxide-based materials, which are frequently used as interlayer dielectric materials, thereby requiring the deposition of appropriate barrier materials prior to actually filling corresponding trenches and via openings with the copper-based material. Although silicon nitride and related materials have excellent diffusion blocking capabilities, using silicon nitride as an interlayer dielectric material is less than desirable due to the moderately high dielectric constant, which may result in a non-acceptable performance degradation of the metallization system. Similarly, in sophisticated applications, the reduced distance of metal lines requires a new type of dielectric material in order to reduce signal propagation delay, cross-talking and the like, which are typically associated with a moderately high capacitive coupling between neighboring metal lines. For this reason, increasingly so-called low-k dielectric materials are being employed, which generally have a dielectric constant of 3.0 or less, thereby maintaining the parasitic capacitance values in the metallization system at an acceptable level, even for the overall reduced dimensions in sophisticated applications.
Since silicon dioxide has been widely used in the fabrication of microstructure devices and integrated circuits, a plurality of modified silicon oxide-based materials have been developed in recent years in order to provide dielectric materials with a reduced dielectric constant on the basis of precursor materials and process techniques that are compatible with the overall manufacturing process for microstructure devices and integrated circuits. For instance, silicon oxide materials with a moderately high amount of carbon and hydrogen, for instance referred to as SiCOH materials, have become a frequently used low-k dielectric material, which may be formed on the basis of a plurality of precursor materials, such as silane-based materials in combination with ammonium and the like, applied by chemical vapor deposition (CVD) techniques and the like. In other cases, spin-on glass (SOG) materials may be modified so as to contain a desired high fraction of carbon and hydrogen, thereby providing the desired low dielectric constant.
In still other sophisticated approaches, the dielectric constant of these materials is even further reduced by incorporating a plurality of cavities of nano dimensions, also referred to as pores, which may represent gas-filled or air-filled cavities within the dielectric material, thereby obtaining a desired further reduced dielectric constant. Although the permittivity of these dielectric materials is reduced by incorporating carbon and forming a corresponding porous structure, which may result in a very enlarged surface area at interface regions connecting to other materials, the overall mechanical and chemical characteristics of these low-k and ultra low-k (ULK) materials are usually significantly altered compared to conventional dielectric materials and may result in additional problems during the processing of these materials.
For example, as discussed above, the dielectric material is typically provided first and is then patterned so as to receive trenches and via openings, thereby requiring the exposure of the sensitive low-k dielectric materials to various reactive process atmospheres. That is, the patterning of the dielectric material may typically involve the formation of an etch mask based on a resist material and the like followed by plasma-assisted etch processes in order to form the trenches and via openings corresponding to the design rules of the device under consideration. Thereafter, usually cleaning processes may have to be performed in order to remove contaminants and other etch byproducts prior to depositing materials, such as conductive barrier materials and the like. Consequently, at least certain surface areas of the sensitive low-k dielectric materials are exposed to the influence of a reactive plasma in processes such as etch processes, resist strip processes performed on the basis of an oxygen plasma, wet chemical reagents in the form of acids, aggressive bases, alcohols and the like, which may thus result in a certain degree of surface modification or damage. For instance, the low-k dielectric materials are typically provided with a hydrophobic surface in order to hinder the incorporation of OH groups and the like, which represent polarizable groups that may therefore efficiently respond to an electrical field, thereby significantly increasing the resulting permittivity of the surface portion of the material.
When exposing the sensitive low-k dielectric material to reactive process atmospheres, such as a plasma-assisted etch process, aggressive wet chemical reagents and the like, the hydrocarbon groups of the hydrophobic surface area are greatly removed, thereby generating a plurality of non-saturated silicon bonds at the surface and within a certain interface layer, which may have a thickness of several nanometers to 20 nm or more. Consequently, after patterning the sensitive low-k dielectric material, for instance for forming via openings and trenches therein, any exposed surface areas thereof and in particular the inner sidewall surface areas of the openings comprises a significant amount of dangling silicon bonds, which may efficiently react with moisture in the ambient atmosphere after the reactive etch process. Consequently silanol groups (OH) react with the non-saturated silicon bonds and thus form a surface layer comprising highly polarized molecules, which in turn additionally result in adsorption of moisture and the like. Consequently, the interface layer of the sensitive low-k dielectric material may comprise a significant amount of polarizable molecules, which in turn results in a significantly enhanced dielectric constant locally at the openings. This may thus lead to a significant parasitic capacitance of metal lines and vias to be formed on the basis of the previously etched openings. Furthermore, the silanol groups and the additional moisture adhering thereto may influence the further processing, for instance when forming a barrier material and the like, which may result in a less reliable electromigration behavior and the like. Consequently, the general reduction of the dielectric constant of the dielectric materials of complex metallization layers may be offset to a pronounced degree due to the incorporation of silanol groups and moisture at an interface between the dielectric material and the metal lines and vias. Hence, great efforts are being made in providing silicon oxide-based low-k dielectric materials while avoiding or at least reducing the surface modifications caused by the patterning of the sensitive dielectric materials and the subsequent exposure to moisture-containing process ambients. In some conventional strategies, it has been suggested to selectively remove the damaged surface layer of the low-k dielectric materials on the basis of appropriate etch strategies, which may particularly remove the polarizable molecules without unduly damaging the hydrophobic nature of the resulting new surface layer. In this case, however, appropriate etch recipes have to be applied without exposure of the resulting structure to any further aggressive process atmospheres in order to maintain the hydrophobic nature of the newly formed surface until the conductive barrier material and the like is deposited. This requires significant efforts in finding appropriate etch recipes, thereby contributing to increased process complexity. Additionally, the material removal results in an increase of the critical dimensions of the metal lines and vias, which may be undesirable in view of enhanced packing density, since the increased critical dimensions have to be taken into consideration when designing the metallization system under consideration. On the other hand, a reduction of the initial critical dimension may not be compatible with the patterning capabilities in the metallization layer under consideration. Consequently, in other alternative approaches, the hydrophobic nature and thus the dielectric properties may partially be restored by applying a so-called low-k repair by means of silylation. In this case, appropriate chemicals react with the previously generated silanol groups on the dielectric surface, wherein the hydrogen atom is substituted by an appropriate functional group including methyl groups, thereby providing a hydrophobic surface area for the further processing of the device and also reestablishing to a certain degree the low dielectric constant locally at the surface areas. In order to initiate the silylation reaction, it is, however, necessary to expose the device to the ambient atmosphere after the plasma-based patterning process in order to saturate the dangling silicon bonds with the silanol groups. Although a plurality of the hydrogen atoms of the silanol groups may be replaced with appropriate functional groups containing hydrogen and carbon, nevertheless the oxygen atom of the silanol group may remain in the damaged surface region, thereby generally affecting the dielectric and chemical characteristics of the dielectric material. Moreover, typical silylation chemicals comprise molecules of large size, which may not efficiently diffuse into the surface of the damaged interface layer, which may thus result in moderately long process times and/or a reduced degree of substitution of the hydrogen atoms with methyl group containing species. Furthermore, as discussed above, upon exposing the damaged sensitive low-k dielectric material to an ambient atmosphere in order to form the silanol groups, water molecules may also adsorb to the polarizable molecules and corresponding hydrogen bonds may strongly reduce the reaction of the silylation molecules with the silanol groups due to the steric hindrance caused by molecules adhering to the damaged surface.
Consequently, although a surface treatment with chemical reagents may provide a certain degree of restoration of the hydrophobic surface conditions of porous silicon dioxide-based dielectric materials, the resulting interface formed between the restored dielectric material and the metal material may still have an increased dielectric constant and chemical characteristics may differ from the characteristics of the low-k dielectric material that has initially been formed.
In view of the situation described above, the present disclosure relates to manufacturing techniques in which silicon dioxide-based low-k dielectric materials may be exposed to plasma treatments or other reactive process techniques, while avoiding or at least reducing the effects of one or more of the problems identified above.